Fabrication and mechanical properties of high-dispersion-treated carbon nanofiber/alumina composites

High-dispersion-treated carbon nanofibers (CNFs) were used to fabricate uniformly-dispersed CNFs-alumina composites with enhanced mechanical properties. The treatment was effective in obtaining dense and uniform composites. The composites containing 0.4–0.8 wt % CNFs were densified to a relative density of more than 99% by vacuum sintering and subsequent hot isostatic pressing, and those containing 1.6–2.5 wt % CNFs were densified to full density by plasma activated sintering. The maximum bending strength of the composites (1050 MPa) was approximately the same as the bending strength of monolithic alumina (1079 MPa). The maximum fracture toughness of the composites was 5.9 MPa·m0.5, which was a 69% increase compared with the fracture toughness of monolithic alumina (3.5 MPa·m0.5). Fracture toughness (KIC) increased rapidly with a decrease in alumina grain size (G), and we found that the relationship could be expressed by the following equation: KIC = (k1/G2) + k2 (where k1 and k2 are constants).